Sensor device and a signal amplification device of a small detection signal provided by the sensor

ABSTRACT

An amplification device of a small detection signal with a multi-element sensor is provided wherein the amplification factor of the operational amplifier can be made variable and also high accuracy offset cancellation and the reduction of noise in low frequency range is possible. A first switching element  31  selectively connects the normal phase input terminal  14  of the operational amplifier  12  either to the reference potential  60  or to the output terminal  2   a - 2   n  of the sensor element  1   a - 1   n . The inverse phase input terminal  15  is connected to the reference potential  60  through the resistor  41  and at the same time to the output terminal  16  through the resistor  42, 43 . Also one terminal  52  of the capacitor  51  is connected to the output terminal  16  of the operational amplifier  12 , the other terminal  53 , is connected to the output buffer  13 , a second switching element  32  is arranged to selectively connect the terminal  53  to the reference potential  60 . Each resistor  42,43  is selectively short circuited by switching element  33,34  to change the resistance value thus varying the amplification factor of the operational amplifier  12.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an amplification device of a small detection signal with a sensor element, which amplifies a output signal of the sensor element by a operational amplifier. More specifically, the present invention relates to an amplification device of a small detection signal having sensor elements, such as a plurality of thermopile type infrared sensor elements, outputting a small detection signal of DC and low frequency range, which requires a means to prevent the noise in the low frequency range.

2. Related Background Art

The recent high integration and low power consumption CMOS technology contributes largely to the miniaturization and power consumption reduction of the sensor devices. However, in the amplifier configured with differential amplification circuit such as CMOS operational amplifier, the input offset voltage caused by the threshold voltage variation is amplified to create the output offset voltage at the output terminal. The output offset voltage (referred to as “offset voltage” hereafter) becomes large enough, for example when CMOS operational amplifier is used in the amplification circuit in the infrared sensor that outputs a small detection signal of DC and low frequency, to push the amplifier out of linear operational range and may even create the clip status. Also especially when the small detection signal of DC and low frequency is input, the countermeasure against the noise in the low frequency range becomes a further problem in addition to the above offset voltage.

Up to now, the compensation method is known wherein a plurality of infrared sensor elements and the compensation elements having the same electrical characteristics but its signal value do not change even with infrared input are arranged contiguously. The offset value of the amplification circuit is obtained by operating a compensation element with a scanning means so that the output of sensor element is compensated, based upon the above obtained offset value. Please see Japanese Unexamined Patent Application Publication No. H9-218090.

However, the above described compensation method relates to offset voltage and the amplification factor of the amplification circuit is constant. Therefore, if the output of the sensor element is not uniform, there is inconvenience that the output signal after amplification may be too small or too large. The object of the present invention is to dissolve the above inconvenience and to provide an amplification device of the small detection signal with a multi-element sensor wherein the offset voltage of the multi-element sensor can be cancelled and the amplification factor of the operational amplification in the amplification circuit can be adjusted according to the output signal level of each sensor element.

The reference H06-45875 (Japanese Unexamined Patent Application Publication) discloses the switched capacitor amplifier circuit where a capacitor is formed between the inverting input terminal of an operational amplifier and reference voltage. For the cancellation actuation of an offset voltage, the capacitor becomes the load of said operational amplifier only at the time of cancellation actuation of said offset voltage. However, in the amplifier circuit disclosed by this reference, since variation arises to each capacitor resulting from the manufacturing process, it is difficult to fully cancel the voltage of offset. Furthermore, the noise in a low frequency cannot be reduced.

The reference H06-54118 (Japanese Unexamined Patent Application Publication) discloses a driving device of an image sensor having a following characteristic feature. FIG. 7 shows the driving device of the image sensor shown in the reference. The signal line L(1) receives an information signal detected by the photo detector of image sensors, and positive phase input terminal of the operational amplifier 21 is connected to this signal line. The output terminal of the operational amplifier 21 is connected to the positive input terminal of an operational amplifier 22 through an analog switch 24 and a capacitor 25. The analog switch 26 and the capacitor 27 are connected in juxtaposition between the positive input terminal (point A) and a ground potential. The output terminal of an operational amplifier 22 is connected to the positive input terminal of an operational amplifier 23 through an analog switch 28 and a capacitor 29. The analog switch 30 and the capacitor 31 are connected in juxtaposition between this positive input terminal (point B) and the ground potential. A switch 4 (1) is connected between the positive input terminal of the operational amplifier 21 and the ground potential. An analog switch 5 (1) is connected between the output terminal of an operational amplifier 23, and an output line OUTL. The operation of this circuit is explained as follows. At the time t1, capacitors 25 and 29 will be charged by setting analog switches 24, 26, 28 and 30 to ON. The electrical potential difference VC25 between terminals of capacitor 25 serves as VC25=20(Vin+Vos1). The electrical potential difference VC29 between terminals of capacitor 29 serves as VC29=20Vos2. (in this equality, Vin is input voltage, Vos1 and Vos2 are the input offset voltage of the operational amplifier 21 and 22, respectively). Next, at the time t2, the potential accumulated in signal-line L (1) will be discharged by setting analog switches 24, 26, 28 and 30 to OFF and the switch 4 (1) to ON. Then, at the time t3, by setting analog switches 24 and 28 to ON, electrical potential VA of point A will serves as VA=(20Vos1−Vc25)/2+Vos2=−Vin+Vos3, and electrical potential VB of point B will serves as VB=(20(−10Vin+Vos2)−Vc29)/2+Vos3=−100Vin+Vos3. (in this equality, Vos3 is input offset voltage of the operational amplifier 23). Thereby, the input offset voltage Vos1 and Vos2 of the operational amplifier 21 and 22, respectively, are canceled, and then the whole circuit gain can be made into 100 times. Concerning to the circuit disclosed in this reference, the positive terminals of the operational amplifiers 22 and 23 will be in a floating state from time t2 to time t3. Therefore, the electric charge charged by the capacitor 25 and 29 must continue being held during the period (time t2 to t3). The capacitors 27 and 29 formed for holding of the electric charge, and capacity with the big capacitor 25, 27, 29 and 31 is needed. Moreover, since it is dependent on the accuracy of capacitors 27 and 31, the accuracy of offset voltage is difficult for controlling with high precision. Furthermore, since the analog switches 24 and 28 are formed on the signal line of the operational amplifier, the noises by switching operation and the transmission delay by parasitism capacity arise.

SUMMARY OF THE INVENTION

In order to achieve the above object, the amplification device of a small detection signal according to claim 1 of the present invention comprises; a first operational amplifier which amplifies said detection signal inputted from a positive input terminal; a first switching element which changes alternatively the signal inputted into the positive input terminal to either the detection signal or a reference potential; a capacitor which the potential of one terminal is determined in conjunction with an output of the first operational amplifier, and obtains an output of said amplification device from the other terminal of the capacitor; and a second switching element which is connected between the reference potential and the other terminal of the capacitor; thus cancels the offset voltage and reduces the noise in low frequency range.

According to the amplification device of a small detection signal of the claim 1 of the invention, the capacitor is arranged at the output of the operational amplifier and the second switching element at the output side is turn off earlier than the first switching element at the input side thus maintaining offset voltage without being affected by input of operational amplifier. Therefore, the offset voltage is cancelled after the output signal is amplified. This allows canceling the offset voltage as well as reducing the noise at low frequency range with two switching elements and one capacitor of small capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the block diagram of the first embodiment showing the configuration of the amplification device of a small detection signal with a multi-element sensor.

FIG. 2 is the block diagram of the second embodiment showing the configuration of the amplification device of a small detection signal with a multi-element sensor.

FIG. 3 is the block diagram of the third embodiment showing the configuration of the amplification device of a small detection signal with a multi-element sensor.

FIG. 4 is the block diagram of the fourth embodiment showing the configuration of the amplification device of a small detection signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiment will be explained based upon the attached drawings. At first, the first embodiment will be explained based upon the block diagram of FIG. 1. As is shown in FIG. 1, an amplification device of a small detection signal with a multi-element sensor comprises a sensor part A and an amplification part B1. The sensor part A comprises a multi-element sensor elements 1 a-1 n including n thermopile type infrared sensor elements (referred to “sensor element” hereafter). An output terminals of one side of 2 a-2 n of each sensor element 1 a-1 n are connected to an output terminal 5 of the multi-element sensor through each switching element 4 a-4 n. The output terminal 5 is connected to a normal phase input 14 of an operational amplifier 12 through a first switching element 21 of the amplification part B1. And the output terminals 3 a-3 n of the other side of the above each sensor element 1 a-1 n are connected to a reference potential 60 through an output terminal 6 of the multi-element sensor.

Each switching element 4 a-4 n is, for example, a three terminals analog switch made of MOS transistor. Each switching element 4 a-4 n is selected alternately to become on and the output terminal of sensor element 1 a-1 n corresponding to the on-status switching element 4 a-4 n is sent to the amplification part B1. The operation of the above each switching element 4 a-4 n is controlled by a clock signal from a control circuit that is not shown in the FIG. 1.

Next, the amplification part B1 will now be explained. An inverse phase input 15 of the operational amplification 12 is connected to the reference potential 60 through a resistor 41, and it is connected to an output terminal 16 of the above operational amplifier 12 through serially connected each resistor 42 and 43. A third and a fourth switching element 33 and 34 that short circuit the both terminal are connected between the both terminals of the above each resistor 42 and 43. Above operational amplifier 12 is the normal phase operational amplifier configured with MOS transistors and the amplification circuit of the embodiment of the present invention functions as the normal phase operational amplifier. In the present specification, the amplification circuit means the part that is configured with the component that exists in the amplification part except an output buffer that will be described later.

The output terminal 16 of the operational amplifier 12 is connected to an input terminal 17 of an output buffer 13 through a capacitor 51. The output buffer 13 is also configured with MOS transistors and it provides the high impedance input terminal 17. With a second switching element 32 open, a terminal 53 of the capacitor 51 that is connected to the input terminal 17 is able to keep the charge immediately before as far as the input of the operational amplifier 12 does not change. The the above output buffer 13 can be of any type as far as it provides the high input impedance 17 and the other configuration can be selected appropriately. Also an output terminal 18 of the above output buffer 13 is the output terminal to the following stage that is not shown in FIG. 1 and the connection point Y of the input terminal 17 of the above output buffer 13 and the other terminal 53 of the above capacitor 51 is connected to the reference potential 60 through the above second switching element 32.

A switching element 31 connects the normal input terminal 14 of operational amplifier selectively either to one of the output terminal 2 a-2 n of the sensor element 1 a-1 n where the switching element 4 a-4 n is on or to the reference potential 60 and for example, it is a three terminal analog switch configured with MOS transistor. The second switching element 32 is also a three terminal analog switch configured with MOS transistor and the operation of the above switching element 31,32 are controlled by the clock signal from the control circuit that is not shown in FIG. 1.

The operation of the above configured embodiment of the present invention will be now explained. Supposing that the switching element 4 a is selected to be on by the control circuit that is not shown, the purpose is to modify the amplification factor of the operational amplifier 12 according to the output level of the sensor element 1 a corresponding to switching element 4 a. Supposing here the resistance value of the resistor 41,42,43 to be respectively R1, R2, R3, then the resistance value affecting the amplification factor of operational amplifier 12 is 1+(R2+R3)/R1 when the switching elements 33 and 34 is off as shown in FIG. 1, 1+R2/R1 when only the fourth switching element 34 is on to short circuit the resistor 43, and 1+R3/R1 when only the third switching element 33 is on to short circuit the both terminals of the resistor 42.

Hereafter, the situation will be explained where the resistance value 1+R2/R1 is selected to be optimum to adjust the amplification factor of operational amplifier 12 according to the output signal level of the above switching element 4 a so that the third switching element 33 turns off and the fourth switching element 34 turns on. In this situation, at first, the first and the second switching element 31 and 32 are connected to the reference potential 60 and the normal input terminal 14 of operational amplifier 12 and the connection point Y of each terminal 53,17 is also connected to the reference potential 60 (the first status).

Supposing that the reference potential 60 to be 0V, the potential of output terminal 16 of the above operational amplifier 12 in the first status to be Vx1, the potential of the connection point Y to be Vy1, the input offset voltage of operational amplifier 12 to be Vos, the resistance value of resistor 41 and 42 to be respectively R1,R2, the capacitance of capacitor 51 to be C and the electric charge in capacitor 51 charged in status 1 to be AQ, the following equation (1) is obtained. Vy 1−Vx 1=−Vos×(1+R 2/R 1)=□Q/C  (1)

That is, the charge that corresponds to the output voltage error created by input offset voltage Vos of the operational amplifier 12 is charged in capacitor 51.

Then the second switching element 32 turn off to disconnect the connection point Y from the reference potential 60 (second status). In this second status, the input terminal 17 of the output buffer 13 is in high impedance status and the terminal 53 of the capacitor 51 becomes floating by making the second switching element 32 to be off. At the same time the normal input terminal 14 of the operational amplifier 12 is kept connected to the reference potential 60 and the potential of the output terminal 16 does not change so that the charge in capacitor 51 that is charged in first status is held. Because the voltage held between a terminal 52 and 53 of the capacitor 51 is only the error voltage by the input offset voltage Vos, the capacitance of the capacitor 51 can be relatively small.

Then the first switching element 31 is disconnected from the reference potential 60 and is connected to the output terminal 5 (third status). The normal input terminal 14 of the operational amplifier 12 is connected, instead of the reference potential 60, to the output terminal 2 a of the sensor element 1 a corresponding to the switching element 4 a selected to be in on status, and the output signal of the above sensor element 1 a is input to the operational amplifier 12. Here the charge of capacitor 51 in first and second status is conserved. Supposing the potential of the output 16 in third status to be Vx3, the potential of the connection point Y to be Vy3 and the potential of output signal of the above sensor element 1 a to be Vo3, we obtain the following equation (2): Vy 3−Vx 3=Vy 3−(Vo 3+Vos)×(1+R 2/R 1)=□Q/C  (2)

Inserting the equation (1) into the equation (2) we obtain the following equation (3): Vy 3=Vo 3×(1+R 2/R 1)  (3) With this equation (3) it is shown that in the third status, the output voltage Vy3 appear at the connection point Y that cancelled the error voltage Vos×(1+R2/R1) of the output voltage created by the input offset voltage Vos of the operational amplifier 12, that is the offset voltage. And this output voltage Vy3 is output through the output buffer 13 from the output terminal 18 to the next stage that is not shown.

The amplification circuit of the present embodiment can amplify, with the repetition of the first status, the second status and the third status, the output signal from sensor elements 1 a-1 n that is selected by the on status of the switching element 4 a-4 n without being affected by the input offset voltage of the operational amplifier 12. The transition time between the first status and the second status may be decided according to the necessary charge up time for the capacitor 51 to the voltage corresponding to the error Vos×(1+R2/R1). The transition time between the second status and the third status may be decided by the enough time to switch off the second switching element 32. The transition time between the third status and the first status may be decided by the time for the error voltage charged in the capacitor 51 to cancel the offset voltage and this is appropriately decided according to the next stage circuit, for example, the third status may be switched to the first status according to the sampling timing of the next stage circuit.

As is shown the above, in the amplification circuit of the embodiment of the present invention, the offset voltage caused by the input offset voltage Vos is once kept between the terminal 52 and 53 of the capacitor 51 and then the output signal of the sensor element 1 a-1 n is input to the operational amplifier 12, the above offset voltage kept in the capacitor 51 is deducted to perform the offset cancellation. Therefore in this embodiment, the offset cancellation is possible with simple configuration with one capacitor 51 and two switching elements 31 and 32 that makes possible the miniaturization of the amplification circuit, the reduction of the consumption power and at the same time the high precision offset cancellation and the noise reduction at the low frequency range. Also turning on or off each third and fourth switching element 33 and 34 according to the output signal level from the sensor elements 1 a-1 n changes the resistance value, and thereby the amplification factor of the operational amplifier 12 can be varied.

Subsequently, the second embodiment will be explained based upon the block diagram of FIG. 2. In the first embodiment, the operation of the amplification circuit has been explained referring to the normal operational amplification device but in this embodiment, the amplification circuit is the inverse operational amplification device and the same component element will be explained using the same symbol. As is shown in FIG. 2, the normal input terminal 14 of the operational amplifier 12 in the amplification part B2 is connected to the reference potential 60. The inverse input terminal 15 is connected to the first switching element 31 through the resistor 41 and also connected to the output terminal 16 through each serially connected resistors 42 and 43. The the above first switching element 31 connects through the resistor 41 the inverse input terminal 15 selectively either to the reference potential 60 or the output terminal 5 of the sensor part A. The other configuration is the same as that of the first embodiment and it will not omitted herein.

In this embodiment as in the first embodiment, each switching element 33,34 is controlled to be on or off by the clock signal from the control circuit that is not shown, the resistors 42, 43 connected between the inverse input terminal 15 of the operational amplifier 12 and the output terminal 16 are selected to change the total resistance value so that the amplification factor of the inverse operational amplification circuit that the operational amplifier 12 configures can be varied. Supposing here the resistance value of the resistor 41, 42, 43 to be respectively R1, R2, R3, then the resistance value affecting the amplification factor of operational amplifier 12 is (R2+R3)/R1 when the switching elements 33 and 34 is off as shown in FIG. 2, R2/R1 when only the fourth switching element 34 is on to short circuit the resistor 43 and R3/R1 when only the third switching element 33 is on to short circuit the both terminals of resistor 42.

Consequently the operation of the amplification circuit of the above configured embodiment will be explained and the switching element 4 b is supposed to be selectively switched on by the control circuit that is not shown. And the amplification factor of the operational amplifier 12 adapting to the output level of the sensor element 1 b corresponding to the switching element 4 b is supposed to be obtained by the resistance value (R2+R3)/R1 and each switching element 33 and 34 is in off status (see FIG. 2). In this situation, at first, the first and the second switching element 31 and 32 are connected to the reference potential 60 and the inverse input terminal 15 of operational amplifier 12 and the connection point Y of each terminal 53,17 is also connected to the reference potential 60 (the first status).

Supposing that the reference potential 60 to be 0V, the potential of output terminal 16 of the above operational amplifier 12 in the first status to be Vx1, the potential of the connection point Y to be Vy1, the input offset voltage of operational amplifier 12 to be Vos, the resistance value of resistor 41, 42 and 43 to be respectively R1, R2 and R3, the capacitance of capacitor 51 to be C and the electric charge in capacitor 51 charged in status 1 to be AQ, we obtain the following equation (4). Vy 1−Vx 1=Vos×{(R 2+R 3)/R 1}=ΔQ/C  (4)

That means, as the same as the first embodiment, the charge corresponding to the output voltage error created by input offset voltage Vos of the operational amplifier 12 is charged in the capacitor 51.

Then the second switching element 32 is switched off to disconnect the connection point Y from the reference potential 60 (second status, see FIG. 2). In this second status, the input terminal 17 of the output buffer 13 is in high impedance status and the terminal 53 of the capacitor 51 becomes floating by making the second switching element 32 to be off. Also the inverse input terminal 15 of the operational amplifier 12 is kept connected to the reference potential 60 and the potential of the output terminal 16 does not change so that the charge in capacitor 51 that is charged in first status is held. Here because the voltage between the terminal 52 and 53 that is kept by the capacitor 51 is only the error voltage by the input offset voltage, the capacitance of the capacitor 51 can be relatively small.

Then the first switching element 31 is disconnected from the reference potential 60 and is connected to the output terminal 5 of the element of the multi-element sensor (third status, see FIG. 2). With this status, the inverse phase input terminal 15 of the operational amplifier 12 is connected, instead of the reference potential 60, to the output terminal 2 b of the sensor element 1 b which is in on status according to the switching element 4 b, and the output signal of the above sensor element 1 b is input into the operational amplifier 12. Here the charge of capacitor 51 in first and second status is conserved. Supposing the potential of the output 16 in third status to be Vx3, the potential of the connection point Y to be Vy3 and the potential of output signal of the above sensor element 1 b to be Vo3, we obtain the following equation (5): Vy 3−Vx 3=Vy 3+(Vo 3+Vos)×{(R 2+R 3)/R 1}=ΔQ/C  (5)

Inserting the equation (4) into the equation (5) we obtain the following equation (6): Vy 3=−Vo 3×{(R 2+R 3)/R 1}  (6) With this equation (6) it is shown that in the third status, the output voltage Vy3 appears at the connection point Y that cancelled the error voltage Vos×{(R2+R3)/R1} of the output voltage created by the input offset voltage Vos of the operational amplifier 12, that means the offset voltage. This output voltage Vy3 is output through the output buffer 13 from the output terminal 18 to the next stage circuit that is not shown. And in this embodiment, the input and the output of the operational amplifier 12 are inverted with the reference potential 60 as the center.

As is described the above, the operational effect of this embodiment that functions the inverse phase amplification device in the amplification circuit is the same as that of the first embodiment using the normal phase amplification device in the amplification circuit. The cancellation of the offset voltage and the reduction of the noise in low frequency range are possible and also the amplification factor of the operational amplifier 12 can be varied according to the output signal level of the selected sensor element 1 a-1 n.

Subsequently, the third embodiment will be explained based upon the block diagram of FIG. 3. In this embodiment, a plurality of operational amplifiers is connected in cascade to adapt for the treatment of a smaller signal and to get larger amplification factor. It functions as the normal phase input operational amplification in the amplification circuit as is in the first embodiment. The same components having the same configurations as the first and the second embodiment will be explained with the same symbols as those of the first and the second embodiments. As is shown in FIG. 3, in the amplification part B3, the normal phase input terminal 14 of the operational amplifier 12 is connected to the output terminal 5 side of the multi-element in the sensor part A through the first switching element 31. The inverse phase input terminal 15 is connected to the reference potential 60 through the resistor 41 and at the same time is connected to the output terminal 16 through the serially connected each resistor 42 and 43. To both terminals of the above each resistor 42 and 43, the third and fourth switching element 33 and 34 that short circuit the both terminals are connected.

The output terminal 16 of the operational amplifier 12 is connected to a normal input terminal 20 of an output buffer 19 through the capacitor 51. This operational amplifier 19 is the same as the above operational amplifier 12 and its output terminal 22 is connected to the input terminal 17 of the output buffer 13 through a capacitor 54 that is same as the above capacitor 51. An additional second switching element 37 similar to the second switching element 32 is connected between the reference potential 60 and the point Z that connects the terminal 56 of the capacitor 54 and the input terminal 17 of the output buffer 13. And the above terminal 56 of the capacitor 54 can keep the charge immediately before by switching off the second switching element 37 as far as the input of the operational amplifier 19 does not change. The output terminal 18 of the above output buffer 13 becomes the output terminal to the next circuit that is not shown in FIG. 3.

And the normal phase input terminal 20 of the operational amplifier 19 is connected to the reference potential 60 through the second switching element 32. An inverse phase input terminal 21 is connected to the reference potential 60 through a resistor 44 and at the same time is connected to an output terminal 22 through the serially connected each resistor 45,46 to configure the normal phase operational amplification device. In addition, to both terminals of the above each resistor 45 and 46, a fifth and a sixth switching element 35 and 36 that short circuit the both terminals are connected. Above operational amplifier 19 has the high impedance input terminal 20 and when the second switching element 32 is off, it makes the terminal 53 of the capacitor 51 floating, thus the charge in the capacitor 51 is kept constant as far as the input of the pre-stage of operational amplifier 12 does not change. And the configurations of the above each resistor 45, 46 and each switching element 35, 36 are the same as those of the above each resistor 42, 43 and each switching element 33, 34.

Also as same as the above each embodiment, each switching element 33, 34 and 35 36 is on/off controlled by the clock signal from the control circuit that is not shown thus making the resistor 42, 43 and 45, 46 to be selectable that is connected between the inverse phase input terminal 15, 21 and the output terminal 16, 22 of the operational amplifier 12, 19. Accordingly the amplification factor of each normal phase amplification device that consists of the operational amplifier 12 and 19 becomes variable by changing the resistance value. The variable resistance value for the operational amplifier 12 is the same as that described in the first embodiment. As for the operational amplifier 19, supposing the resistance value of the resistor 44, 45, 46 to be R4, R5, R5, then the resistance value related to the amplification factor of the operational amplifier 19 is 1+(R5+R6)/R4 in the status shown in FIG. 3. It is 1+R6/R4 when only the fifth switching element 35 is switched on to short circuit the resistor 45 and 1+R5/R4 when only the sixth switching element 36 is switched on to short circuit the both terminal of the resistor 46.

Hereafter the operation of the above configured embodiment of the present invention will be explained. Supposing that the switching element 4 n is selected to be on and the amplification factors of the operational amplifier 12, 19 according to the output level of the sensor element in corresponding to the switching element 4 n are obtained by the respective resistance value 1+(R2+R3)/R1 and 1+(R5+R6)/R4 with each switching element 33,34,35,36 in off status (see FIG. 3). In this status, at first, the first-switching element 31 and the second switching element 32, 37 are connected to the reference potential 60. Also the normal phase input terminal 14 of the operational amplifier 12, the normal input terminal 20 (the connection point Y) of the operational amplifier 19 and the connection point Z of each terminal 56 and 17 are connected to the reference potential 60 (first status).

With this configuration, by the same effect as described in the first embodiment, the error voltage corresponding to the input offset voltage of the operational amplifier 12 is created between the terminal 52 and 53 of the capacitor 51. Also by the same effect, the error voltage corresponding to the input offset voltage of the operational amplifier 19 is created between the terminals 55 and 56 of the capacitor 54.

Then the second switching element 37 of the final stage (second stage in this embodiment) operational amplifier 19 is switched off (No.1 of the second status, see FIG. 3). With this configuration, the terminal 56 of the capacitor 54 becomes floating, and as the normal phase input 20 of the operational amplifier 19 is connected to the reference potential 60, the error voltage corresponding to the input offset voltage of the operational amplifier 19 is kept in between the terminals 55 and 56 of the capacitor 54.

After that, the second switching element 32 of the first stage operational amplifier 12 is switched off (No.2 of the second status, see FIG. 3). With this configuration, the terminal 53 of the capacitor 51 becomes floating, and as the normal phase input 14 of the operational amplifier 12 is connected to the reference potential 60, the error voltage corresponding to the input offset voltage of the operational amplifier 12 is kept between the terminals 52 and 53 of the capacitor 51.

Then the first switching element 31 is disconnected from the reference potential 60 (see FIG. 3) and connected to the output terminal 2 n of the sensor element in through the output terminal 5 and the switching element 4 n being in on status. With this configuration, the normal phase input terminal 14 of the operational amplifier 12 is connected to the above output terminal 2 n instead of the reference potential 60 and the output signal of the above sensor in is input to the above operational amplifier 12. And as the result, by the same effect as described in the first embodiment, the output voltage of the operational amplifier 12 of which offset voltage is cancelled appears at the connection point Y. This output voltage is input to the operational amplifier 19 and by the same effect as described in the first embodiment, the output voltage of the operational amplifier 19 of which output voltage is cancelled appears at the connection point Z. Then this output voltage is output through the output buffer 13 from the output terminal 18 to the next stage that is not shown.

As described in this embodiment, even with the multistage operational amplifier based upon the amplification circuit of the first embodiment, the offset of each operational amplifier can be cancelled and also the noise in low frequency range can be reduced with the following steps: at first connecting the first switching element 31 and each second switching element 32,37 to the reference potential 60 to charge each stage capacitor 51,54 with the offset voltage of each stage amplifier 12, 19; switching off each second switching element 32,37 starting from the last second switching element 37 to keep the charge of each stage between the terminals 52,53 and 55,56 of capacitor 51,54; finally switching the first switching element 31 to the output terminal 2 a-2 n of the sensor element 1 a-1 n.

The present invention is not limited to the above each embodiment and for example in the third embodiment, the stage is two but it can be three or more, as far as the operational amplifier 19 at the second stage or more has the high impedance input terminal 20, 21. And it is also possible to configure the multistage amplification circuit with the inverse phase operational amplifier as described in the second embodiment. Further, the sensor elements 1 a-1 n are not limited to a thermopile infrared sensor as far as they output a small detection signal of DC and low frequency.

The fourth embodiment of the invention will now be explained with reference to FIG. 4. An amplification device of the small detection signal with a multi-element sensor consists of a sensor part A, an amplification part B and an output hold part C.

The sensor part A comprises a multi-element sensor having, for example, n thermal type infrared sensor elements (refer to “sensor element” hereafter) 1 a-1 n. The one side output terminal 2 a-2 n of each sensor element 1 a-1 n is connected to the normal phase input terminal 99 of the first operational amplifier 100 through switching SW1 consisting of the switching elements 3 a-3 n. And the normal input terminal 99 is connected to the reference potential Vref through the switching element SW2. Each switching element 3 a-3 n is, for example, 3 terminals switching element configured by MOS transistor and only the selected switching element is turned on by the control circuit 106. Accordingly, the output signal of the sensor element 1 a-1 n corresponding to the switching element 3 a-3 n that becomes on is output to the amplification part B.

Next, the amplification part B will be explained. The inverse phase input terminal 98 of the first operational amplifier 100 is connected to the reference potential Vref through the resistor R1 and at the same time, is connected to the output terminal 111 of the first operational amplifier 100 through the resistor R2. The first operational amplifier 100 is the amplifier configured by MOS transistor of which amplification factor is decided by the resistance value of the resistor R1 and R2 and the amplification factor is set to be double in this embodiment. The amplification factor of 2-4 times is suitable for the first operational amplifier 100 but 2 times is selected to minimize the noise effect from the amplifier located in latter stage. The first operational amplifier 100 is the normal phase input operational amplifier and the output 111 is connected to the inverse phase input terminal of the own operational amplifier 100 through the resistor R2. And the output terminal 111 is connected to the inverse phase terminal 112 of the second operational amplifier 101 through the resistor R3 and its normal input terminal 113 is connected to the reference potential Vref. The output terminal of the second operational amplifier 101 is connected to the own inverse phase input terminal 112 through the resistor R4. The second operational amplifier 101 is the amplifier configured by MOS transistor, as same as the first operational amplifier 100, of which amplification factor is decided by the resistance value of the resistor R3 and R4, and in this embodiment, the amplification factor is set to be 100 times. It is effective to make the second operational amplifier 101 to be inverse phase input operational amplifier in order to avoid the wraparound input of the signal noise from other circuit. The output terminal of the second operational amplifier 101 is connected to the normal phase input terminal 117 of the first buffer 102 through the capacitor C1. And the normal phase input terminal 117 of the first buffer 102 is connected to the reference potential Vref through the switching element SW3. The first buffer 102 is also configured by MOS transistor and its normal phase input terminal 117 is normally in high impedance status. And the terminal 115 of the capacitor C1 that is connected to the normal phase input terminal 117 is, with switching element SW3 off, able to hold the data immediately before, as far as the input from the second operational amplifier 101 does not change. Also as its input terminal is in high impedance status, the first buffer 102 is effective to lower the impedance and to send data to the third operational amplifier 103 of the latter stage. The output terminal 118 of the first buffer 102 is connected to the inverse phase input terminal 116 of the own buffer 102 and is connected at the same time to the inverse phase input terminal 119 of the third operational amplifier 103 through the resistor R5. Also the inverse phase input terminal 119 of the third operational amplifier 103 is connected to the output terminal of own third operational amplifier 103 through the serially connected resistors R6, R7, R8. Between the both terminals of each resistors R6, R7, R8, the switching element SW6, SW7, SW8 short circuiting each of the both terminals are provided. These switching elements SW6, SW7, SW8 are arranged to make the amplification factor of the third operational amplifier 103 variable and with controlling the on-status of these switching elements, the feedback resistance value is set to be able to obtain a desired amplification factor. Also, the normal phase input terminal 120 is connected to the reference potential Vref. It is effective to make the third operational amplifier 103 to be inverse phase input operational amplifier in order to avoid the wraparound input of the signal noise from other-circuits. The output terminal of the third operational amplifier 103 is connected to the normal phase input terminal 124 of the second buffer 104 through the capacitor C2. And the normal phase input terminal 124 of the second buffer 104 is connected to the reference potential Vref through the switching element SW4. The second buffer 104 is also configured by MOS transistor and its normal phase input terminal 124 is normally in high impedance status. And the terminal 122 of the capacitor C2 that is connected to the normal phase input terminal 124 is, with switching element SW4 off, able to hold the data immediately before, as far as the input from the third operational amplifier 103 does not change. Also as its input terminal is in high impedance status, the second buffer 104 is effective to lower that impedance and to send data to the latter stage circuit.

The output terminal 125 of the second buffer 104 is connected to the inverse phase input terminal 123 of the own second buffer 104 and at the same time is connected to the normal phase input 127 of the third buffer 105 of the output hold part C through the switching element SW5. Also between the normal input terminal 127 of the third buffer 105 and the reference potential Vref, a capacitor C3 is arranged and during the on-status of the switching element SW5 the capacitor C3 is charged according to the potential of the output terminal 125 of the third-buffer-104. This charge is maintained during the off-status of the switching element SW5. When the switching element SW5 turn on at the next step, the potential corresponding to the next data appear at the output terminal 125 and the potential of the capacitor C3 is set again according to that output.

Subsequently, the operation of the amplifier of above configured embodiment of the present invention will be explained. Now, the operation will be explained with the condition where the switching element 4 n is supposed to be on by the control circuit 106; the operational factor of the third operational amplifier 103, adapting to the output level of the sensor 1 n corresponding to the switching element 4 n, is obtained by the resistance value of 1+(R6+R7+R8)/R5; each switching element SW6, SW7, SW8 is in off status.

From this status, at first, turn on the first switching element SW2, the second switching element SW3 and the third switching element SW4 to connect the normal phase input terminal 99 of the first operational amplifier, the normal phase input terminal 117 (connection point Y) of the first buffer and the normal phase input terminal 124 (connection point Z) of the second buffer to the reference potential Vref (status 1). With this connection, the output voltage error caused by the input offset voltage Vos of the first operational amplifier 100 is amplified by the amplification factor of the second operational amplifier and charged into the capacitor C1. The charge that corresponds to the output voltage error created by input offset voltage Vos2 of the third operational amplifier 103 is charged in capacitor C2.

Then, switch off the switching element SW4 and cut off the connection between the connection point Z and the reference potential Vref (the second status). With this configuration, the terminal 122 of the capacitor C2 becomes floating, and as the normal phase input 120 of the third operational amplifier 103 is connected to the reference potential Vref, the error voltage corresponding to the input offset voltage of the third operational amplifier 103 is held between the terminal 121 and 122 of the capacitor C2. Then switch off the switching element SW3 and cut off the connection between the connection point Y and the reference potential Vref (the third status). In this status, the terminal 115 of the capacitor C1 becomes floating. With this condition, because the normal phase input terminal 99 of the first operational amplifier 100 is connected to the reference potential Vref, the error voltage corresponding to the input offset voltage appears at the output terminal 111 of the first operational amplifier 100. And the error voltage amplified by the amplification factor of the second operational amplifier is held in the capacitor C1.

Subsequently, the switching element SW2 is turned off to cut off the connection between the normal phase input terminal 99 of the first operational amplifier 100 and the reference potential Vref and the switching element SW1 is turned on to connect the normal input terminal 99 to the out put terminal 2 n of the sensor element 1 n (the forth status) With this step, the output of the sensor element in is amplified by the first and second operational amplifier 100,101 wherein the offset voltage of the first operational amplifier 100 is cancelled and the voltage appears at the connection point Y. The output voltage of the connection point Y is input to the first buffer 102 and the third amplifier 103 where the offset voltage is cancelled as well and the voltage appears at the connection point Z. In this embodiment too, the offset voltage of the first and the second operational amplifier can be cancelled and at the same time the noise at the low frequency range can be reduced. 

1. An amplification device of a small detection signal provided by the sensor element comprising: a first operational amplifier which amplifies said detection signal inputted from a positive input terminal; a first switching element which changes alternatively the signal inputted into said positive input terminal to either said detection signal or a reference potential; a capacitor which the potential of one terminal is determined in conjunction with an output of said first operational amplifier, and obtains an output of said amplification device from the other terminal of said capacitor; and a second switching element which is connected between said reference potential and said other terminal of said capacitor.
 2. An amplification device according to claim 1, further comprising: a first resistor connected between said reference potential and a negative input terminal of said first operational amplifier; and a second resistor connected between said negative input terminal and an output of said first operational amplifier.
 3. An amplification device according to claim 2, further comprising: a third resistor connected in series to said second resistor between said negative input terminal and said output of said first operational amplifier; a third switching element which is provided for connecting between the terminals of said second resistor; and a forth switching element which is provided for connecting between the terminals of said third resistor.
 4. An amplification device according to claim 1, 2 or 3, further comprising: a control circuit for controlling said first switching element and said second switching element, so as to connect between said positive input terminal of said first operational amplifier and said reference potential, and between said other terminal of said capacitor and said reference potential firstly, then between said other terminal of said capacitor and said reference potential is disconnected electrically, after that, the signal inputted into said positive input terminal is changed from said reference potential to said detection signal.
 5. An amplification device according to claim 1, further comprising: a second operational amplifier having a positive input terminal, a negative input terminal and an output terminal, and said positive input terminal is connected to said capacitor via a switching element, said negative input terminal is connected to said reference potential via a resistor element, and said output terminal is connected to said negative input terminal via a resistor.
 6. An amplification device according to claim 1, further comprising: an inverting operational amplifier having a positive input terminal, a negative input terminal and an output terminal, and said positive input terminal is connected to said reference potential, said negative input terminal is connected to the output of said first operational amplifier via a resistor element, and said output terminal is connected to said negative input terminal via a resistor.
 7. A sensor device according to claim 1, said first switching element comprising: a signal line connected to said positive input terminal of said first operational amplifier; an auxiliary switching element connected between said signal line and said sensor element; and a main switching element connected between said signal line and said reference potential.
 8. A sensor device having a plurality of sensor elements and an amplification device for amplifying a small detection signal of alternatively selected sensor element, said amplification device comprising: a first operational amplifier which amplifies said detection signal inputted from a positive input terminal; a first switching element which changes alternatively the signal inputted into said positive input terminal to either a reference potential or said detection signal that is provided by one of said plurality of sensor elements; a capacitor which the potential of one terminal is determined in conjunction with an output of said first operational amplifier, and obtains an output of said amplification device from the other terminal of said capacitor; and a second switching element which is connected between said reference potential and said other terminal of said capacitor.
 9. A sensor device according to claim 8, further comprising: a first resistor connected between said reference potential and a negative input terminal of said first operational amplifier; and a second resistor connected between said negative input terminal and an output of said first operational amplifier.
 10. A sensor device according to claim 9, further comprising: a third resistor connected in series to said second resistor between said negative input terminal and said output of said first operational amplifier; a third switching element which is provided for connecting between the terminals of said second resistor; and a forth switching element which is provided for connecting between the terminals of said third resistor.
 11. A sensor device according to claim 8, 9 or 10, further comprising: a control circuit for controlling said first switching element and said second switching element, so as to connect between said positive input terminal of said first operational amplifier and said reference potential, and between said other terminal of said capacitor and said reference potential firstly, then between said other terminal of said capacitor and said reference potential is disconnected electrically, after that, the signal inputted into said positive input terminal is changed from said reference potential to said detection signal.
 12. A sensor device according to claim 8 or 9, further comprising: a second operational amplifier having a positive input terminal, a negative input terminal and an output terminal, and said positive input terminal is connected to said capacitor via a switching element, said negative input terminal is connected to said reference potential via a resistor element, and said output terminal is connected to said negative input terminal via a resistor.
 13. A sensor device according to claim 8, further comprising: an inverting operational amplifier having a positive input terminal, a negative input terminal and an output terminal, and said positive input terminal is connected to said reference potential, said negative input terminal is connected to the output of said first operational amplifier via a resistor element, and said output terminal is connected to said negative input terminal via a resistor.
 14. A sensor device according to claim 8, said first switching element comprising: a plurality of auxiliary switching elements, each of said auxiliary switching elements is connected between a common signal line and one of said sensor elements respectively; and a main switching element which connects said positive input terminal to either said detection signal or said reference potential alternatively.
 15. A sensor device according to claim 8, said first switching element comprising: a plurality of auxiliary switching elements, each of said auxiliary switching elements is connected between a common signal line and one of said sensor elements respectively; and a main switching element connected between said common signal line and said reference potential. 